Method and apparatus for measuring the optical quality of a reflective surface

ABSTRACT

A method and apparatus for inspecting the optical quality of a reflective surface providing for the reflecting of a beam of light off the reflective surface, measuring an intensity of the reflected light at a first distance from said reflective surface, measuring an intensity of the reflected light at a second distance from said reflective surface, and comparing the intensity of the light measured at the two distances to determine the distortion of the reflective surface.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/323,539 filed Sep. 20, 2001, which is herebyincorporated by reference.

Background

[0002] 1. Field of the Invention

[0003] The present invention relates to a method and apparatus formeasuring the optical quality of a reflective surface, for example,measuring the flatness of a sheet of glass that could contain adefective region where the surface is slightly curved.

[0004] 2. Background and Technical Field Of the Invention

[0005] Production of tempered glass consists of heating the material toits softening temperature, then fast chilling, to introduce compressivesurface stresses and increase its strength. In this process, the hotmaterial is supported and moved in and out of the heating chamber by aset of rolls. As result of the combined actions of sag between the rollsand roller eccentricity, the glass sheet deforms slightly, acquiring asurface waviness, also called Roller Wave, as shown in FIG. 1. Wheninstalled in a building, glass exhibiting this waviness will generatedistortion of reflected images and be considered defective.

[0006] Several tools and methods are presently used to inspect temperedglass. With reference to FIG. 1 showing a sheet of glass 10 having awaviness, the simplest measuring tools include a depth gage revealingthe depth w of the wave as a difference between peak 12 and valley 13heights of the glass 10. The depth w of the wave, however, does notfully describe the optical distortion.

[0007] Other methods use optical means to quantify the opticaldistortion. With reference to FIG. 1A, devices such as that in U.S. Pat.No. 3,857,637 to Obenreder measures an angle B of a reflected beam oflight 16 off of the surface 18 of the glass 10 using a beam-positionsensing device 20. The reflected beam comes from a light source LSproviding a beam of light 14 directed at the surface 18. This approachrequires measuring the angle B of reflection at two or more points 22 a,22 b, and recording the variation of the reflected angle B and thedistance d between the measured points 22 a, 22 b, to permit thecalculation of the optical distortion.

[0008] U.S. Pat. No. 5,251,010 to Maltby discloses a method thateliminates the need of measuring the distances d such as thatillustrated in FIG. 1B. Maltby discloses methods whereby two parallelbeams of light, which can be split from a single light source LS withpartial mirrors 23 as shown, separated by a known distance d, arereflected off the inspected surface 18 and sensed by twoposition-sensing devices 20a,20b. As result of the curvature due to theroller wave, these two beams 16a,16b diverge or converge, and the changein the angle B between these beams provides the measure of thedistortion.

[0009] A nearly identical device is described in U.S. Pat. No. 5,122,672to Mansour and U.S. Pat. No. 5,210,592 to Betschneider. The two-beamapproach requires very accurate beam-position detectors and does notaccount for the difficulties in measuring the beam position when thebeam shape becomes irregular as result of the surface curvature. Anotherapproach, described in Redner & Bhat, “New Distortion Measuring MethodUsing Digital Analysis of Projection-Moire Patterns” SAE Transactions,106 (6) 1997, uses the image of a Moire screen projected on a master,forming Moire fringes that reveal changes in magnification due to localcurvature of the inspected item. The application of this method is alsodescribed in U.S. Pat. No. 5,128,550 to Erbeck. Another method based onmeasurements of the dimensional size of the reflected beam is describedin U.S. Pat. No. 4,585,343 to Schave. In this method, edges of thereflected beam are located using an array of detectors. This method isessentially equivalent in performance to the two-beam method in U.S.Pat. No. 5,251,010 discussed above since the distance between the twoedges of the reflected beam is used to measure angular changes. Morerecently, an approach proposed in U.S. Pat. No. 6,100,990 to Ladewskiuses reflected images of gray-scale patterns. Assuming that the rollerwave is periodic in nature, the distribution of light intensity analysispermits calculation of the optical power of the inspected surface.

[0010] All of the above methods have serious limitations, conceptual orpractical in nature. Common difficulties include the following:

[0011] a) The measured angular deviation B is very small, and thedetection of small changes in the reflected beam position cannot beaccomplished accurately considering that the glass sheet vibrates as itemerges from the tempering furnace, and

[0012] b) The surface curvature deforms the beam of light, making itdifficult to locate its center using a position-sensing device.

[0013] For at least these reasons, a new method and apparatus thatovercomes the limitations of the prior methods and apparatuses isdesirable.

[0014] One objective of the invention is to provide for measuringoptical distortion more accurately, eliminating the reflected beamsposition-sensing detectors used in the above described devices.Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention.

SUMMARY OF THE INVENTION

[0015] The present invention provides a method for inspecting theoptical quality of a reflective surface, such as flat sheet of glass.The method can include the following steps: (a) reflecting a beam oflight off of the reflective surface; (b) measuring an intensity of thereflected light at a first distance from said reflective surface; (c)measuring an intensity of the reflected light at a second distance fromsaid reflective surface, where the first distance is different than saidsecond distance; and (d) comparing the light intensity measured inparagraph b with the light intensity measured in paragraph c todetermine the distortion of the reflective surface.

[0016] A device for carrying out the method is also provided. The deviceincludes a light source for directing a beam of light towards thereflective surface from which the beam of light is reflected. A firstphotodetector measures the intensity of the reflected beam of light at afirst distance from the reflective surface. A photodetector, preferablya second photodetector, measures the intensity of the reflected beam ata second distance which is further from the reflective surface than themeasurement made by the first photodetector. A readout system receivesthe measurements made by photodetectors and indicates the opticalquality.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a schematic diagram showing roller-wave distortion;

[0018]FIG. 1A is a schematic view of a known optical measuring method;

[0019]FIG. 1B is a schematic view of another known optical measuringmethod;

[0020]FIG. 2A is a schematic view of a reflection of light from a flatsurface for purposes of demonstrating the principles of the invention;

[0021]FIG. 2B is a schematic view of a reflection of light from a curvedsurface for purposes of demonstrating the principles of the invention;

[0022]FIG. 2C is a schematic view of a reflection of light from a curvedsurface being measured for purposes of demonstrating the principles ofthe invention;

[0023]FIG. 3 is a schematic view of a preferred embodiment of theinvention;

[0024]FIG. 4 is a schematic view of another preferred embodiment of theinvention; and

[0025]FIG. 5 is a schematic view illustrating the use of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0026] As discussed above and illustrated in FIG. 1, glass undergoingthe heat-tempering process deforms out of its plane, forming a wavypattern called “Roller Wave”. The reference letters and definitionsrelevant to FIG. 1 are defined as follows:

[0027] L=wavelength

[0028] w=peak-to-valley depth

[0029] R=radius of curvature (Note: F=R/2)

[0030] D=Optical power (Note: D=1/F)

[0031] The peaks 12 and valleys 13 of the waviness are spaced by adistance L, called wavelength. Light reflection of the convex andconcave regions introduce a distortion of reflected images, similar tothe action of cylindrical mirrors. The image distortion depends of thelocal radius of curvature R and of the focal length F, related to R.Typically, a nearly perfect glass sheet has a radius of curvature largerthan 100 meters, but a defective item could have locally a radius 10meters or smaller. The optical effect of the local curvature is bestdescribed by the optical power D, related to the focal length and radiusof curvature by:

D=1/F=2/R

[0032] The distortion introduced by a wavy surface can be evaluatedquantitatively, measuring the optical power D. For example, measuringpeak-to-valley depth w and the wavelength L yields the distortion, asshown in the Strainoptic Technologies Inc. Instruction Manual forMaintenance and Use of RWG Roller Wave Gage, and in the U.S. patentsdiscussed above, and which describe the use of a reflected beam oflight, or a pair of beams, with position-sensing detectors measuring theangular deviation of these beams. An equivalent result is obtained bythe Schave reference, which described a device detecting the position ofedges of a reflected beam.

[0033] The present invention eliminates the need for reflected beamposition-sensing detectors as used in devices described above andprovide means for measuring optical distortion more accurately. Instead,the reflected beam's divergence or convergence is measured directly,using the energy density principle, which is now described withreference to FIGS. 2A,2B,and 2C.

[0034] As illustrated in FIG. 2A, when a cylindrical, collimated beam oflight 14 from a collimated light source LS reflects off of a perfectlyflat surface 18 the reflected beam 16 remains collimated, and neglectinglight losses in the air, will illuminate a target placed in its way withthe same luminous intensity, regardless of the target distance (e.g.,x₁, and x₂) from the glass surface 18. Thus, with reference to FIG. 2A,the illumination at a target placed in the beam 16 at a distance x₁,will have the same illumination as at a target placed in the beam 16 ata distance x₂, or, put another way, the energy of the beam per unit area(the light intensity=I/a which can be expressed in units of watts/area)remains constant since target illumination is independent of thedistance x from the surface 18.

[0035] With reference to FIG. 2B, as a result of surface waviness, thereflected beam 16 will acquire a diverging or converging angle B(diverging being illustrated in the FIG. 2B), covering an increasing ordecreasing area a, as the distance x from the inspected surface 18increases. Since the luminous energy I will be now spread over anincreasing or decreasing area a, the light intensity (I/a) becomes afunction of the distance x from the inspected surface 18, and also ofthe divergence angle B. Thus, with reference to FIG. 2B illustrating adiverging beam 16, the luminous energy at a distance x₂ is spread overan area a₂ that is larger than the area a₁, at x₁, where the luminousenergy is spread over a smaller area. Measurement of the light intensityat two points R₁ and R₂, located at a distance x₁ and x₂ from theinspected surface 18, provides sufficient information to calculate theoptical distortion. A sample calculation is illustrated with referenceto FIG. 2C.

[0036] In FIG. 2C, a converging light beam 16 reflected from acylindrical reflecting surface 18 is shown. The measuring of lightintensity using a photodetector detector 28 is performed near the centerof the reflected beam 16 at two points, R₁ and R₂, where the variationdue to the vibration and to the motion of the measured item isminimized, making the measuring apparatus more accurate and reproduciblethan the beam position sensing devices. The term photodetector as usedherein is any device capable of converting light intensity into anelectric signal, and includes photo electric sensors, photo diodes. Asshown in the equations below, derived from the FIG. 2C, the opticaldistortion can be computed simply from the measured photoelectriccurrents i₁ and i₂ produced by the photoelectric detectors 28, i₁, andi₂ being the current produced by the photoelectric detectors 28 locatedin areas a, and a₂ respectively. $\begin{matrix}{D = {\frac{1}{F} = \frac{B}{A_{o}}}} & (1)\end{matrix}$

[0037] where A₀=the area of surface 18 being sampled (illuminated);$\begin{matrix}{B = \frac{a_{1} - a_{2}}{d}} & (2)\end{matrix}$

[0038] where a₁, and a₂=the illuminated areas at R1 and R2 respectively,and B is the convergence angle;

[0039] Measured photoelectric currents i₁ and i₂ at points R₁ and R₂respectively by the photodetectors 28 are proportional to the sourceintensity I₀:$i_{1} = {{{\frac{I_{o}}{a_{1}}\&}\quad i_{2}} = \frac{I_{o}}{a_{2}}}$

[0040] Change in the size of the illuminated areas is related to thechange in measured intensities: $\begin{matrix}{{a_{1} - a_{2}} = {Z = {I_{o}( {\frac{1}{i_{1}} - \frac{1}{i_{2}}} )}}} & (3)\end{matrix}$

[0041] From the geometry it can also be shown that $\begin{matrix}{A_{o} = {{a_{o} + {H \cdot B}} = {a_{o} + {\frac{H}{d}( {a_{1} - a_{2}} )}}}} & (4)\end{matrix}$

[0042] where a₀ is the aperture of the measuring system and H is thedistance of the aperture from the surface 18.

[0043] Combining eq. 1, 2, 3 & 4 yields: $\begin{matrix}{D = {\frac{a_{1} - a_{2}}{{a_{o}d} + {H( {a_{1} - a_{2}} )}} = \frac{Z}{{a_{o}d} + {HZ}}}} & (5)\end{matrix}$

[0044] For high-sensitivity of detection, H is small and d is large,making H/d negligible. The above equation thus reduces to:$D = \frac{Z}{a_{o}d}$

[0045] In essence, the new method permits measuring the opticaldistortion of a reflecting surface by simply measuring a differentialoutput of two photodetectors.

[0046] Apparatus and Method for Measuring Optical Distortion

[0047] To better illustrate the principle of the new method andapparatus, reference is made to FIGS. 3 and 4. It is understood thatthese drawings are simplified, to better illustrate the methodology, andthat any person skilled in the art can produce a large variety ofoptical element arrangements to accomplish essentially the same result.

[0048] Shown in FIG. 3 is a preferred embodiment having a light sourceLS such as an incandescent lamp, a laser beam, a tip of a fiber-opticcable channeling the luminous energy from a remote source, or othersuitable source. An incandescent point source of about 20 watts issuitable. The light source is placed in a focal plane of a lens 30,directing a collimated beam 14 of light toward the inspected region(sample area) of the glass sheet 10. In practice, as result of the sizeof the light source, the illuminating beam 14 will be slightly divergentor convergent. A divergence adjustment is provided by a focusing device32, such as an adjustable housing, adjusting the distance between thelight source LS and the lens 30. A diffuser 34 can be provided betweenthe light source LS and the lens 30. A diffused light source can provideequivalent results since the surface of the diffuser functions as aninfinite number of point sources located in the same plane, each onebehaving as a single point.

[0049] The incidence angle “A” (see FIG. 4) between the incident beam 14and the normal 36 to the surface 18 of the glass 10 is preferably verysmall, as shown in the FIG. 3. To obtain a small angle A, a beamsplitter 38 is mounted to receive the illuminator beam 14 and directthis beam in a direction towards the surface 18 and perpendicular to it.An alternative design, shown in FIG. 4, incorporating a large angle “A”is equally effective, and offers an equivalent solution. Thisconfiguration eliminates light losses due to the presence of the beamsplitter 38.

[0050] The light beam 14 is then reflected from the surface 18, thereflected beam being illustrated with reference numeral 16. To permitmeasurement of the light intensity at two points R₁ and R₂, distant x₁and x₂ respectively from the inspected surface 18, another beamsplitter, or a beam-divider cube 40 (a 50-50 divider being preferred)intercepts the reflected beam 16, dividing the reflected beam into twobeams 42 and 44 which reach the R₁ and R₂ points after traveling adistance x₁, and x₂ respectively, where x₁=ONR₁, and x₂=ONR₂, as shownin FIGS. 3 and 4 (the distance x₁, e.g., being the distance from point Oto N to R₁). Since the sensitivity of detection is proportional to thedistance d between the interception points x₁, and x₂ (see FIG. 2C), itis advantageous to make the distance d as large as practical. To measuredistortion between 20 and 150 mdpt (millidiopters) typically encounteredwhen inspecting tempered glass, a 50 mm diameter beam 14 can be used,with the distance d between the paths ONR1 and ONR2 about 400 mm. Thedistance ON should be kept as small as practical. It is understood bythose in the art that the distances from the reflective surface 18 atwhich the light intensity measurements are made, e.g., x₁and x₂, is thedistance the light travels from the reflective surface 18 to R₁ and R₂,not necessarily the actual straight line distance from the reflectivesurface 8 to R₁ and R₂. For example, mirrors, in a manner known in theart, can be used to increase the distances x₁ and x₂ without increasingthe actual distance of R₁ and R₂ from the surface 18. The distances x₁and x₂ are the distances the light travels from the surface 18 to R₁ andR₂.

[0051] To measure the light intensity at R₁ and R₂, an aperture mask 46having an opening of area a₀, may be incorporated, to control the sizeof the measured beam, admitting only the central region of the reflectedbeam 16 where the uniformity of the energy distribution is better. Amask 46 having an aperture a₀ slightly smaller than the original beam 14is suitable.

[0052] Masks M₁ and M₂, having apertures a₃ and a₄ respectively, alsopermit selection of the portion of the beam used for the light intensitymeasurements, rejecting peripheral regions that are affected by theglass motion. Aperture a₃ is preferably smaller than a₂ since the lightbeam at the further pont R₂ is spread out more, a suitable a₄ beingabout 25 mm and a suitable a₃ being about the half that size. Colorselective filters F1 and F2 can be used to select a suitable range ofwavelengths, especially when coated glasses are inspected. Addition ofdiffusers 48 and 50 provide an integrating action, further eliminatingan undesired sensitivity to small displacement of the beam center due tosolid-body motion. Condenser lenses 52 and 54 can be incorporated toimprove the light efficiency of these diffusers. The light intensity atR₁ and R₂, over the area (a, and a₂ in FIG. 2C) limited by the masks M₁and M₂ is measured using suitable photodetectors PS₁, and PS₂, such assilicon photo diodes or any other suitable device. It is seen that eachphotodetector can be housed in an assembly 51 with the other relatedcomponents, for example, photodetector PS₁ is in an assembly with MaskM1, filter F1, diffuser 48, and condenser 52.

[0053] A readout system 55 in communication with the photodetectors PS₁and PS₂ through wires 57 is provided to analyze the measurements anddisplay the results in a desired format. For example, the readout system55 can include a photoelectric amplifier/readout instrument 5 wherebyphotoelectric currents i₁ and i₂, proportional to the light intensity atR₁ and R₂ are displayed by the photoelectric amplifier/readoutinstrument 56. In addition, the readout system 55 can include adifferential amplifier 58, having an adjustable gain for calibration,which receives the output of the detectors PS₁and PS2, measuring anddisplaying the difference Z between the light intensities i₁, at thepoint R₁ and i₂ at the point R₂. The analogue output of thephotodetector/amplifier can be furthermore digitized, and connected to acomputer 60 as part of the readout system, for data storage, graphicdisplay of information and statistical presentation.

[0054] Using simple relations of geometrical optics, illustrated in FIG.2C, the measured optical distortion D is related to the measureddifference of light intensities Z, by the following equation:

D−Z/(H*Z+a ₀ *d)

[0055] Where H, a₀, and d are geometrical factors defined by distancesx₁, x₂ and by the position of the instrument above the inspectedsurface, and H is shown in FIGS. 3 and 4. For small values of H,preferably about 50 mm, the above equation reduces to:

D=Z/(a ₀ *d)=K*Z

[0056] showing a direct proportionality between the optical distortion Dand the measured output Z.

[0057] The device of the present invention should preferably becalibrated before use. For example, this can be done by using the deviceto measure a defect free flat piece of glass and adjusting the device sothat the difference between the two currents i₁ and i₂ is zero toindicate an absence of optical distortion, e.g., adjusting the amplifier56. The proportionality constant K=1/a₀*d is measured in a calibrationexperiment, using a surface with a known radius of curvature, the devicethen being calibrated, for example, by adjusting the gain of thedifferential amplifier 58, or by a suitable software procedure forcalibration.

[0058] The present invention also permits measurement of curvature inarbitrarily selected planes of the surface 18 being measured. Thisselection can be made using masks M₁ and M₂, shown in FIGS. 3 and 4,with a slit shaped aperture that permits sensing of light intensityvariations due to divergence or convergence in a plane parallel to theslit only. For example, when sensing a cylindrical roller wavecurvature, the sensitivity to the curvature in the plane perpendicularto the roller wave can be decreased or increased, depending on the testobjectives, by placing the slit parallel to the direction of the rolls.

[0059] As shown in FIG. 3, the various components can be mounted withina housing 62 configured to minimize background light from entering thehousing and interfering with the measurements. The light source LS canbe mounted in an adjacent housing 64, such as an electrical box, mountedon the outside of the housing 62 to allow convenient access to the lightsource, such as a bulb, for easy changing without having to open up themain housing 62 An opening in the side of the housings 62 and 64 betweenthe two allows the light to enter the main housing 62. A window 66,preferably of flat glass, allows the light beam 14 to leave the housing62, reflect off surface 18, and reenter the housing 62 for measurement.The window 66 is preferably angled slightly as shown to eliminateundesirable reflections of light. Adjacent the beam splitter 38 is ananti-reflecting surface 68 to eliminate stray light that may passthrough the beam splitter 38 from the light source LS.

[0060] It is seen that the apparatuses of FIGS. 3 and 4 can inspect theoptical quality or distortion of a reflective surface 18 at a singlepoint or region of the sheet 10 (off-line, or can inspect a selectedline along the sheet 10 as the sheet 10 is moved relative to theapparatus (on-line). For example, illustrated in FIG. 5 is an apparatus70 similar to that described with reference to FIG. 3 positioned above aroller bed 72. The sheet 10 to be inspected moves on the roller bed 72beneath the device 70 as the device 70 inspects the surface 18 along aline on the surface 18. Alternatively, the sheet 10 can be placed on aflat table 74 for inspection at specific points or regions on thesurface 18.

[0061] While particular embodiments of the invention are describedherein, it is not intended to limit the invention to such disclosure andchanges and modifications may be incorporated and embodied within thescope of the appended claims.

1. A method for inspecting the optical quality of a reflective surface,said method comprising: (a) reflecting a beam of light off of thereflective surface; (b) measuring an intensity of said reflected lightat a first distance from said reflective surface; (c) measuring anintensity of said reflected light at a second distance from saidreflective surface, said first distance being different than said seconddistance; and (d) comparing the light intensity measured in paragraph bwith the light intensity measured in paragraph c to determine theoptical quality of the reflective surface.
 2. The method of claim 1wherein the beam of paragraph (a) is provided by an incandescent lightsource.
 3. The method of claim 2 wherein the beam of paragraph (a) isprovided by a collimated incandescent light source.
 4. The method ofclaim 1 wherein the measuring functions of paragraphs (b) and (c) arecarried out with a photodetector.
 5. The method of claim 1 wherein themeasuring functions of paragraphs (b) and (c) each comprise the step ofgenerating an electric signal representative of the light intensitybeing measured.
 6. The method of claim 5 wherein the step of generatingan electric signal representative of the light intensity measured iscarried out with at least one photodetector capable of producing anelectric current proportional to the light intensity being measured. 7.The method of claim 4, further comprising the step of providing a maskhaving an aperture through which said light passes to saidphotodetector, said aperture having a geometry chosen to control adirectional sensitivity of the optical quality being determined.
 8. Themethod of claim 1 wherein step d includes the step of calculating theoptical distortion.
 9. The method of claim 1 further comprising the stepof splitting the reflected light into at least two light beams, a firstof said two light beams being measured in step b, a second of said twolight beams being measured in step c.
 10. The method of claim 9 whereinstep b and step c are carried out at the same time.
 11. A method forinspecting reflective surfaces that could contain a defective regionwhere the surface is slightly curved, resulting in changes of reflectedlight direction and a distortion of reflected images, said methodcomprising: (a) directing a beam of light towards the reflectivesurface; (b) reflecting said beam of light off of the reflectivesurface; (c) measuring an intensity of said reflected light at a firstdistance from said reflective surface; (d) measuring an intensity ofsaid reflected light at a second distance from said reflective surface,said second distance being further from said reflective surface thansaid first distance; and (e) determining an optical quality of thereflective surface using the measurements of steps c and d.
 12. Themethod of claim 11 wherein step e is carried by calculating the opticaldistortion using the measurements of steps c and d.
 13. The method ofclaim 11 wherein steps c and d are carried by producing an electricalsignal indicative of the light intensity.
 14. A device for inspectingthe optical quality of a reflective surface, said device comprising: alight source for directing a beam of light towards the reflectivesurface such that said beam of light is reflected from said surface; afirst photodetector for measuring the intensity of the reflected beam oflight, said first photodetector disposed to make said measurement at afirst distance from said reflective surface; a second photodetector formeasuring the intensity of the reflected beam of light, said secondphotodetector disposed to make said measurement at a second distancewhich is further from said reflective surface than said first distance;and a readout system in communication with said first and secondphotodetectors for indicating the optical quality of the reflectivesurface.
 15. The device of claim 14 further comprising a beam splitterpositioned to split the reflected beam into a first beam for measurementby said first photodetector, and a second beam for measurement by saidsecond photodetector.
 16. The device of claim 14 wherein said firstphotodetector is included in an assembly further comprising a maskhaving an aperture, a diffuser, and a condensing lens.
 17. The device ofclaim 14 wherein said readout system comprises an amplifier forincreasing the photodetector output.
 18. The device of claim 14 whereinsaid light source comprises an incandescent bulb and a collimating lens.19. The apparatus of claim 14 whereby the collimated beam illuminatesthe inspected surface at an angle other than normal.
 20. The apparatusof claim 14, wherein said readout system includes a computer.
 21. Theapparatus of claim 14 further comprising a beam splitter positioned todirect the beam of light from the light source towards the reflectivesurface.
 22. The apparatus of claim 14 further comprising a maskdisposed between said reflective surface and said first and secondphotodetectors, said mask having an aperture for controlling the amountof light passing there through.